Chapter 6 Methods.pptx

Introduction to Java Programming and
Data Structures
Twelfth Edition
Chapter 6
Methods
Copyright © 2020 Pearson Education, Inc. All Rights Reserved

Opening Problem
Find the sum of integers from 1 to 10, from 20 to 30, and
from 35 to 45, respectively.

Problem (1 of 2)
int sum = 0;
for (int i = 1; i <= 10; i++)
sum += i;
System.out.println("Sum from 1 to 10 is " + sum);
sum = 0;
for (int i = 20; i <= 30; i++)
sum += i;
sum = 0;
for (int i = 35; i <= 45; i++)
sum += i;

Problem (2 of 2)

Solution
MethodDemo

Defining Methods (1 of 2)
A method is a collection of statements that are grouped
together to perform an operation.
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Define a method Invoke a method
int z = max(x, y);
actual parameters
(arguments)

Defining Methods (2 of 2)
A method is a collection of statements that are grouped
together to perform an operation.
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
Define a method Invoke a method
int z = max(x, y);
actual parameters
(arguments)
method
signature

Method Signature
Method signature is the combination of the method name
and the parameter list.

Formal Parameters
The variables defined in the method header are known as
formal parameters.

Actual Parameters
When a method is invoked, you pass a value to the parameter.
This value is referred to as actual parameter or argument.

Return Value Type
A method may return a value. The returnValueType is the data type of
the value the method returns. If the method does not return a value, the
returnValueType is the keyword void. For example, the
returnValueType in the main method is void.

Calling Methods (1 of 2)
Testing the max method
This program demonstrates calling a method max to return
the largest of the int values
TestMax

Calling Methods (2 of 2)
pass the value of i
pass the value of j

Trace Method Invocation (1 of 10)

CAUTION
A return statement is required for a value-returning method.
The method shown below in (a) is logically correct, but it
has a compilation error because the Java compiler thinks it
possible that this method does not return any value.
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else if (n < 0)
return –1;
}
(a)
Should be
(b)
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else
return –1;
}
To fix this problem, delete if (n < 0) in (a), so that the
compiler will see a return statement to be reached
regardless of how the if statement is evaluated.

Reuse Methods from Other Classes
Note: One of the benefits of methods is for reuse. The max
method can be invoked from any class besides TestMax. If
you create a new class Test, you can invoke the max
method using ClassName.methodName (e.g.,
TestMax.max).

Call Stacks

Trace Call Stack (1 of 10)

void Method Example
This type of method does not return a value. The method
performs some actions.
TestVoidMethod
TestReturnGradeMethod

TestVoidMethod

Passing Parameters
public static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++)
System.out.println(message);
}
Suppose you invoke the method using
nPrintln(“Welcome to Java”, 5);
What is the output?
Suppose you invoke the method using
nPrintln(“Computer Science”, 15);
What is the output?
Can you invoke the method using
nPrintln(15, “Computer Science”);

Pass by Value (1 of 3)
This program demonstrates passing values to the methods.

Modularizing Code
Methods can be used to reduce redundant coding and
enable code reuse. Methods can also be used to
modularize code and improve the quality of the program.
PrimeNumberMethod
• Modular programming (also referred to as modular
architecture) is a general programming concept. It
involves separating a program’s functions into
independent pieces or building blocks, each containing all
the parts needed to execute a single aspect of the
functionality. Together, the modules make up the
executable application program.

Case Study: Converting Hexadecimals to
Decimals
Write a method that converts a hexadecimal number into a
decimal number.
 
 
 
 
ABCD
A *16 ^3 B *16 ^ 2 C *16 ^1 D *16 ^0
A *16 B *16 C *16 D
10 *16 11 *16 12 *16 13 ?

  
   
    
Hex2Dec

Overloading Methods
Overloading the max Method
public static double max(double num1, double
num2) {
if (num1 > num2)
return num1;
else
return num2;
}
TestMethodOverloading

Overloading Methods

Ambiguous Invocation (1 of 2)
Sometimes there may be two or more possible matches for
an invocation of a method, but the compiler cannot
determine the most specific match. This is referred to as
ambiguous invocation. Ambiguous invocation is a
compile error.

Ambiguous Invocation (2 of 2)
public class AmbiguousOverloading {
public static void main(String[] args) {
System.out.println(max(1, 2));
}
public static double max(int num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
public static double max(double num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
}

Scope of Local Variables (1 of 6)
A local variable: a variable defined inside a method.
Scope: the part of the program where the variable can be
referenced.
The scope of a local variable starts from its declaration and
continues to the end of the block that contains the variable.
A local variable must be declared before it can be used.

You can declare a local variable with the same name
multiple times in different non-nesting blocks in a method,
but you cannot declare a local variable twice in nested
blocks.

A variable declared in the initial action part of a for loop
header has its scope in the entire loop. But a variable
declared inside a for loop body has its scope limited in the
loop body from its declaration and to the end of the block
that contains the variable.
public static void method1() {
.
.
for (int i = 1; i < 10; i++) {
.
.
int j;
.
.
.
}
}
The scope of j
The scope of i

int x = 1;
int y = 1;
for (int i = 1; i < 10; i++) {
x += i;
}
for (int i = 1; i < 10; i++) {
y += i;
}
}
It is fine to declare i in two
non-nesting blocks
int i = 1;
int sum = 0;
for (int i = 1; i < 10; i++) {
sum += i;
}
}
It is wrong to declare i in
two nesting blocks

// Fine with no errors
public static void correctMethod() {
int x = 1;
int y = 1;
// i is declared
for (int i = 1; i < 10; i++) {
x += i;
}
// i is declared again
for (int i = 1; i < 10; i++) {
y += i;
}
}

// With errors
public static void incorrectMethod() {
int x = 1;
int y = 1;
for (int i = 1; i < 10; i++) {
int x = 0;
x += i;
}
}

Method Abstraction
You can think of the method body as a black box that
contains the detailed implementation for the method.
Method Header
Method body
Black Box
Optional arguments
for Input
Optional return
value

Benefits of Methods
• Write a method once and reuse it anywhere.
• Information hiding. Hide the implementation from the
user.
• Reduce complexity.

Case Study: Generating Random
Characters (1 of 4)
Computer programs process numerical data and
characters. You have seen many examples that involve
numerical data. It is also important to understand
characters and how to process them.
As introduced in Section 4.3, each character has a unique
Unicode between 0 and FFFF in hexadecimal (65535 in
decimal). To generate a random character is to generate a
random integer between 0 and 65535 using the following
expression: (note that since 0 <= Math.random() < 1.0, you
have to add 1 to 65535.)
(int)(Math.random() * (65535 + 1))

Characters (2 of 4)
Now let us consider how to generate a random lowercase
letter. The Unicode for lowercase letters are consecutive
integers starting from the Unicode for 'a', then for 'b', 'c', ...,
and 'z'. The Unicode for 'a' is
(int)'a'
So, a random integer between (int)'a' and (int)'z' is
(int)((int)'a' + Math.random() * ((int)'z' - (int)'a' + 1)

Characters (3 of 4)
As discussed in Chapter 2, all numeric operators can be
applied to the char operands. The char operand is cast into
a number if the other operand is a number or a character.
So, the preceding expression can be simplified as follows:
'a' + Math.random() * ('z' - 'a' + 1)
So a random lowercase letter is
(char)('a' + Math.random() * ('z' - 'a' + 1))

Characters (4 of 4)
To generalize the foregoing discussion, a random
character between any two characters ch1 and ch2 with
ch1 < ch2 can be generated as follows:
(char)(ch1 + Math.random() * (ch2 – ch1 + 1))

The RandomCharacter Class (1 of 2)
// RandomCharacter.java: Generate random
characters
public class RandomCharacter {
/** Generate a random character between ch1
and ch2 */
public static char getRandomCharacter(char
ch1, char ch2) {
return (char)(ch1 + Math.random() *
(ch2 - ch1 + 1));
}
/** Generate a random lowercase letter */
public static char
getRandomLowerCaseLetter() {
return getRandomCharacter('a', 'z');
}
/** Generate a random uppercase letter */
public static char
getRandomUpperCaseLetter() {
return getRandomCharacter('A', 'Z');
}
RandomCharacter
TestRandomCharacter

The RandomCharacter Class (2 of 2)
/** Generate a random digit character */
public static char
getRandomDigitCharacter() {
return getRandomCharacter('0', '9');
}
/** Generate a random character */
public static char getRandomCharacter() {
return getRandomCharacter('u0000',
'uFFFF');
}
}
RandomCharacter
TestRandomCharacter

Chapter 6 Methods.pptx

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